fix(default-metrics): register metrics alphabetically

[salman-frs]

Default metrics currently follow collector registration order, so the output is not ordered by metric name.

This reorders metric groups and metrics inside multi-metric collectors where needed. Shared collect() callbacks move to the first registered metric so sibling values are populated before the registry reads them.

Collectors such as heapSizeAndUsed register names under more than one prefix, so object order alone cannot produce a globally sorted result. collectDefaultMetrics() therefore sorts only the newly registered default metrics once after registration. Existing custom metrics keep their registration order.

Tests:

• npm test
• bun test-unit

Fixes #810.